The Parts List for the radio (there were 3 different P/N's only one compatable with all 1951/52 Virginian Kaisers) came with the radio installation kit. Since you've indicated the plan is for an updated custom radio package in the vehicle replacing the old stuff (which did not product the sound quality of today's high-quality radios, amplifiers and speakers) let the experts figure this out.
PS: remember, you have a hook-up problem if you haven't switched the car over to negative ground, 12 volt system with most currently available stuff.
Actually, the speakers don't (for the most part) give a damn about what the voltage, wattage, ground is. It is the duty of the radio or stereo amplifier section to produce the appropriate ohm/watt/voltage output to the speakers, and to manage all issues regarding ground. Most speakers are either 8, 4 or 2 ohm. Eight and four are most common in home stereo, four and two most common in car stereo. The pertinent point is how loud will they sound with the same level of input signal. Example...a 4 ohm speaker will sound louder at a given volume level than an 8 ohm speaker at that exact same volume level.
But this is all beside the point to the question I'm asking...which is...what is the size of the actual front speaker in a 51? Is it 6x9, 6x8...some other size?
I want to know what size of modern speaker I need to look for to replace the original, without having to make some modifications to either stuff it in, or brackets to hold a smaller speaker in a larger mounting spot...
For some reason that info is not found in the parts or shop manual...likely because it was an add on accessory...